CDMA multiple carrier paging channel optimization

ABSTRACT

A method of reducing paging channel usage in a CDMA communication system. Each mobile switching centre maintains in its visitor location registry information regarding a paging channel or carrier frequency that each mobile station registered is monitoring. Then, when it comes time to send a page to a mobile station, the mobile switching centre looks up the information and appends it to the page which is sent to all basestation transceivers. The basestation transceivers extract the information and identify the paging channel the mobile station to which the page is addressed is monitoring. The transceivers then broadcast the page message over their air interfaces on the identified paging channel.

FIELD OF THE INVENTION

The invention relates to systems and methods for CDMA (code divisionmultiple access) paging.

BACKGROUND OF THE INVENTION

In CDMA systems, a MS (mobile station) accesses the CDMA system on aparticular carrier frequency. A single carrier frequency occupies 1.25MHz in bandwidth, and provides capacity for a number of calls to proceedsimultaneously in the same cell. Each carrier has associated with it asingle paging channel and a plurality of traffic channels. The channelsof a given carrier occupy the same frequency bandwidth and aredifferentiated from each other by a series of different orthogonalcodes.

When a carrier reaches its capacity limit, it is desirable to increasethe capacity of the system. One way of achieving this is to increase thenumber of carriers to greater than one, thereby creating a multi-carriersystem.

In existing multi-carrier systems, a MS is capable of tuning to only oneof the carrier frequencies at any instant in time. Because of this, theMS is only listening to one of the carrier frequency paging channels. Inorder to page a MS, the page must be sent on the paging channel of allof the carrier frequencies to ensure that it is sent on the pagingchannel the MS is listening to. To do this, the page message must beduplicated for each of the paging channels on which the message is to besent. This results in an inefficient utilization of paging channelresources.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate or mitigate one or more ofthe above identified disadvantages.

According to a first broad aspect, the invention provides in a CDMA(code division multiple access) communications system, a method for amobile switching centre to send messages to mobile stations within itscoverage area, the method first comprising the step of: for each mobilestation, maintaining in a form accessible to the mobile switching centrea paging channel identifier which permits an identification of a pagingchannel the mobile station is monitoring; for each message to be sent toa particular mobile station the method further comprising the steps of:determining on the basis of the paging channel identifier associatedwith the particular mobile station the particular paging channel thatthe particular mobile station is monitoring; and broadcasting themessage on the particular paging channel.

Preferably, each paging channel identifier comprises a band class and achannel number for the associated mobile station to identify the carrierfrequency within the band class that the mobile station is operating ormonitoring.

According to a second broad aspect, the invention provides a CDMAbasestation transceiver for connection to a basestation controller, theCDMA basestation transceiver being adapted to: 1) include a pagingchannel identifier permitting the identification of a paging channelthat a particular mobile station is monitoring in messages to thebasestation controller relating to the particular mobile station; 2)when a message to be sent over the paging channel including said pagingchannel identifier is received from the basestation controller fortransmission to the particular mobile station, identify the pagingchannel the particular mobile station is monitoring on the basis of thepaging channel identifier, and send the message over that pagingchannel.

According to a third broad aspect, the invention provides a CDMAbasestation controller for connection to a mobile switching centre andto a plurality of basestation transceivers, the basestation controllerbeing adapted to: 1) receive a paging channel identifier permitting theidentification of a paging channel that a particular mobile station ismonitoring in messages received from any basestation transceiverrelating to the particular mobile station; 2) include the paging channelidentifier in messages to the mobile switching centre relating to theparticular mobile station; 3) when a message including said pagingchannel identifier is received from the mobile switching centre fortransmission to the particular mobile station, forward the messageincluding said paging channel identifier to the basestationtransceivers.

According to a fourth broad aspect, the invention provides a CDMA mobileswitching centre for connection to a basestation controller and adaptedto: 1) receive a paging channel identifier permitting the identificationof a paging channel that a particular mobile station is monitoring inmessages received from the basestation controller relating to theparticular mobile station; 2) store the paging channel identifier; and3) include the paging channel identifier in messages to the basestationcontroller relating to the particular mobile station.

Advantageously, by monitoring the current frequency that the MS isoperating on, for example by monitoring registration messages or handoffmessages which have been modified so that they include carrier frequencyand bandclass information, the MSC is able to know the current carrierfrequency that the MS is operating on. Using this knowledge, the MSC isable to direct the page message to the paging channel for that carrierfrequency. In a three carrier CDMA system, this would reduce pagingchannel usage by a 2/3 (66%). In general, paging channel capacity wouldbe reduced by (n-1)/n*100%. In addition, time is saved since thebasestation does not need to duplicate the page message for each pagingchannel.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is a block diagram of a conventional CDMA communications system;

FIG. 2 is a message flow diagram for updating VLR (visitor locationregistry) information through mobile registration messaging;

FIG. 3 is a message flow diagram for updating VLR information throughmobile origination messaging;

FIG. 4 is a message flow diagram for updating VLR information throughmobile handoff complete messaging;

FIG. 5 is a message flow diagram for the delivery of a paging channelmessage; and

FIG. 6 is another message flow diagram for the delivery of a pagingchannel message with a retry on different carrier frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIG. 1, a conventional CDMA mobile communicationssystem includes a plurality of MSCs (mobile switching centres) 10,12(only two shown) each connected to a respective plurality of BSCs(basestation controllers) 14,16 (only one shown per MSC), with each BSCbeing connected to a respective plurality of BTSs (basestationtransceivers) 18,20 (only one shown per BSC). Each MSC 10,12 has accessto one or more HLRs 21 (home location registries, only one shown)containing subscriber information for a plurality of subscriber's MSs(mobile stations), for feature capabilities for example. Each MSC 10,12also has a respective VLR (visitor location registry) 23,24 containinginformation regarding all MSs which have registered with the particularMSC. According to network reference models, the VLRs may be internal orexternal to the MSCs. In the illustrated example, a single MS (mobilestation) 22 is shown. It is assumed that there is a record in the HLR 21pertaining to the MS 22. It is also assumed that for the time being MS22 is within the coverage area of MSC 10, and as such MSC 10 will bereferred to as the "current MSC". The MS is said to be within thecoverage area of a given MSC if it is within a cell of a BTS controlledby that MSC.

When the MS 22 registers with the current MSC 10 (through the BTS 18 andBSC 14), a record is created in the VLR 23 in MSC 10, and the MS isassigned by the BTS to a particular band class and channel number. Theband class identifies on which of several band classes the MS 22 isoperating. In an existing CDMA standard the band classes include the 800MHZ band class and the 1900 MHZ band class. The channel numberidentifies the particular carrier within the identified band class thatthe MS has been assigned. Every carrier contains a paging channel forsending messaging and control information. After the MS 22 registers, itbegins monitoring the paging channel associated with its assignedcarrier and band class.

A conventional VLR contains various information for each MS which hasregistered with the corresponding MSC, in some cases including thechannel number for the MS. According to an embodiment of the invention,each record in the VLR also includes an additional field for storing theband class. As described in detail below, the channel number and bandclass information is initialized when the MS registers with the currentMSC, and is updated whenever appropriate. The information is then usedto route paging messages to the appropriate paging channel, therebyeliminating the need to send paging messages on all paging channels toensure that the paging channel being monitored by the MS is covered. Forthe purposes of this description, the information used to identify thepaging channel being monitored will be referred to as a PCI (pagingchannel identifier). For this example, the PCI will consist of thechannel number and band class assigned to a particular MS as thesevalues will permit the paging channel to be identified.

Referring now to FIGS. 2, 3 and 4, message flows for various ways toupdate the VLR with the PCI.

In FIG. 2, VLR update is achieved through mobile registration. Referringto the example of FIG. 1, it is assumed for this example that the MS 22is registering with the current MSC 10. Registration is the processwhich a MS must undergo for example when the MS is turned on in a newcoverage area, or moves from one coverage area to another. When a MSregisters with the system, information about the MS and its operation ispassed to the MSC. This information includes the location of the mobile(cell id or cell identifier), the type of registration, MIN (mobileidentification number), ESN (electronic serial number), whether it isoperating in slotted mode and slot cycle index, etc. In step 1, the MS22 registers with the BTS 18 on a particular band class and carrierfrequency. The BTS with which the MS 22 registers is the BTS for thecell in which the MS is located. How the particular band class andcarrier frequency are selected is outside the scope of this invention.In step 2, the BTS 18 sends a registration message to the BSC 14.According to this embodiment of the invention, this registration messageis modified to include the PCI for the MS 22. In step 3, the BSC 14sends the registration message plus PCI to the current MSC 10. Thecurrent MSC 10 will extract the PCI information and update the VLRrecord for the MS identified by the registration message (according toits ESN and MIN). Following this, the rest of the normal registrationprocess takes place with the HLR 21. Normal registration processes arewell known in the art and will not be described further here.

In FIG. 3, VLR update is achieved through MS origination message flow.An origination occurs when an MS wants to request a call setup over atraffic channel, for example. In step 1, the MS 22 sends an originationmessage to the BTS 18. In step 2, the BTS 18 appends the PCI to theorigination message received from the MS 22 and forwards this to the BSC14. In step 3, the BSC 14 sends the origination message including thePCI to the current MSC 10. The current MSC 10 extracts the PCI from theincoming origination message and updates the relevant record in the VLRwith the information contained in the PCI. In step 4, the current MSC 10sends an assignment request to the BSC 14 instructing the BSC to movethe MS 22 to a traffic channel. At this point, the BSC 14 may assign theMS to a different carrier frequency or band class than was previouslyassigned, and hence to a different PCI. In step 5, a channel assignmentis sent to the BTS 18, and in step 6 this is forwarded to the MS 22. Instep 7 the MS 22 acknowledges receipt of the channel assignment messagefrom the BTS 18, and in step 8 the BTS acknowledges receipt of thechannel assignment message from the BSC 14. Finally, in step 9, anassignment complete message is sent from the BSC 14 to the current MSC10 again containing the PCI for the MS 22. This PCI will be differentfrom the PCI contained in the message sent in the above described step 3if the BSC 14 made a change in the carrier frequency or band class. Thecurrent MSC 10 then updates the VLR with the new band class andfrequency information. The origination processing then continues on asnormal.

VLR update may also be achieved through mobile termination message flow.A termination occurs when a call is set up to the mobile station. Thefirst step in a termination is for the MSC to locate the MS with a pagerequest to the MS. Following this, the message flow for a termination isbasically the same as for an origination with the exception of the factthat the origination messages are replaced by page response messages.

In FIG. 4, VLR update is achieved through a mobile handoff completemessage flow. A hard handoff occurs when a switch is made from a firstBTS (current BTS) to a second BTS (target BTS) and the carrier frequencyand bandclass may change. This is due to the movement of the MS forexample. After a MS is directed to the target BTS from the current BTSvia a handoff direction message (not shown), the mobile switchesfrequencies and reports to the target BTS. It is in the message flowinvolved with reporting to the target BTS that VLR update occurs forthis example. Referring to FIG. 1, it is assumed for this example thatBTS 20 is the target BTS and that BTS 18 is the current BTS. In step 1,the MS 22 sends a "mobile on channel" message to the target BTS 20, thisbeing the new BTS. In step 2, the target BTS 18 forwards the mobile onchannel message to the target BSC 16 for the target BTS including thePCI of the MS 22, BSC 16 being the new BSC. In this example, the targetBSC is different from the current BSC but this may not necessarily bethe case. The target BSC 14 sends a handoff complete message to thetarget MSC 12 including the PCI of the MS 22. The target MSC 12 thenbecomes the new current MSC and creates a new record in its VLR 24 forthe MS 22, and extracts the PCI from the message and stores it in thenewly created record.

When it is necessary to send a page message to a MS, PCI (in thisexample the band class and channel number that the MS is operating on)can be extracted from the VLR and used to direct the page message to aspecific paging channel. This method of paging the MS may be used forany message that is sent on the paging channel. The messages that areeffected by this will depend on the particular CDMA standard beingemployed. In the context of IS-634 for example, these are referred to asA-interface messages and include the Page₋₋ Request message, Feature₋₋notification message, ADDS₋₋ Page message (for short message service),Authentication₋₋ Request message, SSD (shared secret data) UpdateRequest, and the Base Station Challenge Response message.

FIGS. 5 and 6 relate to paging channel message delivery flow. Whilethese examples relate to sending a page request message on the pagingchannel, the message flow may be used for any paging channel messagedelivery to the mobile station. The purpose of a page request message isto determine the location of the MS.

Referring firstly to FIG. 5, before any message flows occur, an initialstep in delivering a page to a particular MS 22 is for the current MSC10 retrieve from its VLR 23 the PCI for the particular mobile station.The first message in the message flow is a page request sent from thecurrent MSC 10 to all its BSCs 14 (only one message shown in messageflow diagram). This page request is extended to include the PCI for theMS 22. Each BSC 14 then sends the page request including the PCI to itsrespective BTSs 18 (only one message shown in the message flow diagram).Each BTS 18 then extracts and removes the PCI from the page requestmessage and broadcasts the page request message over its air interfaceon the paging channel associated with the extracted PCI.

Assuming that the MS 22 received the page request message, it would sendan acknowledgement through normal channels to indicate this fact. If themessage is not acknowledged, then a second attempt to find the MS'slocation is necessary. At this point, to ensure that the MS can befound, the MSC may attempt to page the MS on all carrier frequencies. Inorder to instruct the BSC to send the page message on the pagingchannels associated with all of the carrier frequencies, the MSC sendsthe page request message without the PCI. The absence of thisinformation is an instruction to the BSCs and BTSs to send the messageon all carrier frequencies and band classes. A message flow diagram forthis scenario is shown in FIG. 6. Steps 1,2 and 3 are identical to thosein FIG. 5. The current MSC 10 starts a paging timer when the pagerequest message is sent. If the MS 22 fails to respond before apredetermined time, the timer will expire. Upon expiry of the timer, instep 4 the current MSC 10 sends a new page request message to the BSC 14not including the band class and carrier frequency information. In step5, the BSC 14 forwards this message to the BTS 18. In step 6, the BTS 18broadcasts the page request on all paging channels.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

In addition to restricting page broadcast to a particular pagingchannel, it is also possible that the paging of the mobile station maybe directed to a particular cell, group of cells or all cells.

In the above described example, both a band class and a channel numberare included in a PCI used to identify the paging channel beingmonitored by the MS. Alternatively, any information which would permitthe identification of the paging channel may be used. For example, thePCI may contain a single parameter which uniquely identifies both theband class and the channel number. It may be that in some systems, thereis no concept of band class, but rather simply a plurality of carrierseach with a paging channel. In this case, the PCI need only identify thecarrier.

Furthermore, it is possible that there may be more than one pagingchannel associated with a particular carrier, and that the mobilestation may be monitoring one or more of these paging channels. In thiscase, the PCI need only permit the identification of which pagingchannel(s) the mobile station is monitoring.

In the illustrated embodiment, the PCI is structured such that the BSCis able to distinguish exactly which paging channel the mobile stationis monitoring. Alternatively, the PCI may be structured to only permitthe BSC to narrow down the paging channels the mobile station may bemonitoring to a subset of all possible paging channels. The BSC wouldthen send the page on each of paging channels in the narrowed down setof paging channels. For example, for the case in which there are bandclasses and channel numbers, using a PCI containing only the channelnumber would require the BTS to broadcast the page on the identifiedchannel number for each band class.

Messages transmitted from a mobile station or basestation transceiver toa mobile switching centre often include a basestation identifier fieldwhich identifies the cell to which the message pertains. This field maybe extended to include the PCI in which case the basestation identifiermay be referred to as an extended basestation identifier. Alternatively,the PCI may be appended to other fields of messages, or placed in acompletely new field.

In the illustrated examples, various messages have been identified usingIS-95 or IS-634 standard names, for example. It is to be understood thatthe invention is not limited in its application to systems employingthese particular standards. Any messaging protocol may be used whichallows an MS to convey the PCI to an MSC and vice versa.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a CDMA (code divisionmultiple access) communications system, a method for a mobile switchingcentre to send messages to mobile stations within its coverage area, themethod first comprising the step of:for each mobile station, maintainingin a form accessible to the mobile switching centre a paging channelidentifier which permits an identification of a paging channel themobile station is monitoring; for each message to be sent to aparticular mobile station the method further comprising the stepsof:determining on the basis of the paging channel identifier associatedwith the particular mobile station the particular paging channel thatthe particular mobile station is monitoring; and broadcasting themessage on the particular paging channel; wherein each paging channelidentifier is stored in a visitor location registry.
 2. A CDMA mobileswitching centre for connection to a basestation controller and adaptedto:1) receive a paging channel identifier permitting the identificationof a paging channel that a particular mobile station is monitoring inmessages received from the basestation controller relating to theparticular mobile station; 2) store the paging channel identifier; and3) include the paging channel identifier in messages to the basestationcontroller relating to the particular mobile station; wherein eachpaging channel identifier comprises a band class and a channel numberfor the associated mobile station to identify a carrier frequency withina band class that the mobile station is operating or monitoring; avisitor location registry for storing said paging channel identifier.